The speaker is sitting in an Amsterdam coffeeshop late at night, high and writing in a journal. Physically he feels heavy and slow, but his mind races off in the opposite direction, replaying old grievances and disappointments and trying to "solve" himself, even over problems he says he gave up on years ago. He blames the sativa, which suggests this spiral isn't new to him.
Then the poem shifts outward. A man with bushy hair and a scuzzy beard stares at him, and the speaker assumes he's being watched because of his writing. The joke is that he then does write about the man, though only after the man's behavior began. He quickly turns the suspicion back on himself: maybe he's the paranoid one.
The mood is casual, wry, and a little self-mocking. The one-word last line, "Stoners," shrugs off the whole tangle of thought with humor, as if the speaker sees how absurd his mental loop has been.